1. Field of the Invention
This invention relates to an electrode assembly for a semiconductor device, and more particularly, to an electrode assembly which is bonded between a semiconductor substrate and its mount with a solder consisting mainly of lead and tin.
2. Description of the Related Art
The recent advance in semiconductor technology has brought about the development of various kinds of semiconductor devices. The external electrical connection of any such device has a critical bearing on its performance. While it is essential to employ an electrode for the electrical connection of a semiconductor device to enable it to perform the desired function, the device does not show any satisfactory performance if the lead wire, etc. connected to the electrode causes the diffusion of impurities, or if its electrical connection to the semiconductor is incomplete for the complete transmission of signals.
The external electrical connection of a semiconductor device has usually been achieved by bonding a semiconductor substrate to its mount. The substrate has a chromium or titanium layer and a nickel layer formed one upon the other on its back surface and these electrode layers are bonded to the mount with a solder layer. The chromium or titanium layer forms a layer of contact with the semiconductor, while the nickel layer is a layer for preventing the diffusion of tin from the solder to the substrate. This arrangement makes it possible to maintain the satisfactory electrical connection of the device, while preventing the diffusion of impurity (tin in the solder) to the substrate.
The recent advance in semiconductor technology has, however, created a demand for an electrode construction capable of electrical connection with a lower resistance to a silicon wafer having a high resistivity to obtain a device of higher accuracy in performance. The electrode construction as hereinabove described has too high a contact resistance to achieve any ohmic connection with a satisfactorily low electrical resistance.
It is known that rare-earth metals or rare earth silicides having a low Schottky-barrier height offer a lower contact resistance than chromium or other metals. See e.g. Applied Physics Letters 36(7), 594 (1980) and 38(8), 626 (1981). The rare-earth metals, or silicides are, however, too easily oxidized by reacting with oxygen to be suitable for use in any ordinary electrode construction. If a layer of TiW or tungsten is used as a layer for preventing the diffusion of tin from a solder to a semiconductor, it is possible to prevent the oxidation of a rare-earth metal, or silicide, but the use of any such layer does not make any readily acceptable electrode, since it can only poorly be wetted with the solder.
An electrode assembly comprising layers of a rare-earth metal, nickel and gold may be derived from a known combination of chromium, nickel and gold. It can satisfactorily be bonded with a solder and prevent the diffusion of tin therefrom, but as nickel diffuses into reacts with the rare-earth metal and a semiconductor upon heat treatment at a temperature of about 250.degree. C., it cannot maintain a low specific contact resistivity at varying temperatures, and is, therefore, a construction which is totally unacceptable for practical use.
There has also been proposed an electrode construction which includes an iron-alloy layer for preventing the diffusion of tin. Japanese Patent Application laid open under No. Sho 63-60537 discloses a combination of iron silicide, iron and an Fe-Sn alloy. This construction has, however, the drawback that it cannot maintain a satisfactorily low specific contact resistivity at the interface with a semiconductor and form a contact having a satisfactorily low resistance with a silicone wafer having a high resistivity, though it can effectively prevent the diffusion of tin through iron.